The present invention relates to diagnostic method and apparatus for a gas sensor, and particularly to diagnostic method and apparatus to diagnose whether a gas sensor for sensing an air-fuel ratio of exhaust gas is in an abnormal state or not.
There has been a gas sensor attached to an exhaust passage of an internal combustion engine such as an engine for a vehicle and adapted to sense a concentration of a specific gas component included in an exhaust gas. A detection signal outputted by such a gas sensor (in detail, an sensor element constituting the gas sensor) is sent to an ECU (electronic control unit). The ECU is configured to detect an air-fuel ratio of the exhaust gas on the basis of the received detection signal, and thereby performs an air-fuel ratio feedback control to adjust an injection quantity of fuel for the engine and the like. As such a gas sensor, there is an oxygen sensor for sensing the oxygen concentration in the exhaust gas. Recently, a wideband (full-range) air-fuel ratio sensor adapted to vary its sensor output value linearly according to the oxygen concentration in the exhaust gas has been used in order to achieve a more precise air-fuel ratio feedback control or the like.
In the case that the gas sensor is being used for a long time, there is a possibility that the gas sensor deteriorates with time. Namely for example, a gas-flow hole formed in a protector (in detail, a protector protecting the sensor element by covering a periphery of the sensor element) of the gas sensor or a porous portion guiding the exhaust gas into the sensor element is clogged. If the gas sensor causes such a deterioration, a response of sensor output value according to a variation of the concentration of the specific gas component in the exhaust gas is delayed as compared with a gas sensor which is in not-deteriorated state (i.e., in a normal state).
In the case where the gas sensor has caused such a deterioration, there is a fear that a reduction in operating performance of engine, a reduction in fuel economy, a reduction in cleaning performance of the exhaust gas, or the like is incurred. Hence, it is diagnosed whether or not the gas sensor is in an abnormal state on the basis of the detection signal of the gas sensor. Japanese Patent Application Publication No. H03(1991)-202767 corresponding to U.S. Pat. No. 5,052,361 discloses previously-proposed abnormality diagnostic method and apparatus. In this technique, a deviation between a detection signal outputted by a gas sensor to be diagnosed and a reference value preset outside a value range of a detection signal obtainable by a normal gas sensor is calculated. Then, by comparing the integral of this deviation with a judging value (deterioration reference value) defined as a criterion for deterioration diagnosis, it is diagnosed whether or not the gas sensor is in an abnormal state (deteriorated state).
In this Application Publication No. H03(1991)-202767, as the above reference value for calculating the deviation, two kinds of reference values are provided respectively for the case where a target air-fuel ratio for an air-fuel mixture is in a rich side and for the case where the target air-fuel ratio for the air-fuel mixture is in a lean side. In the normal gas sensor, the value of the detection signal reverses to follow a reversal of the target air-fuel ratio, and varies to sequentially approach the reference value for the rich side and the reference value for the lean side. Accordingly, the deviation between the reference value and the value of detection signal is relatively small. On the other hand, in the gas sensor having some abnormality, the reversal of the detection signal is delayed relative to the reversal of the target air-fuel ratio. Accordingly, the deviation between the detection signal value and the reference value for the rich or lean side is relatively great. Therefore, when calculating the integral of the deviation, this integral has a magnitude according to a deterioration degree of the gas sensor. Thus, the abnormality diagnosis can be performed by comparing this integral of the deviation with the deterioration reference value.